Numerous formats exist for writing data to an optical disc including CD-R, CD-RW, and DVD. In addition, other formats have been proposed that would allow multilevel data (data that includes more than two possible information states per symbol or mark) to be written to an optical disc. As storage density increases and the mark size decreases for various optical data storage schemes, the ability to precisely control the laser waveform used to write data to an optical disc has become more important. In addition to precisely controlling the waveform, it has also become important to provide flexible control so that different waveforms for different write strategies may be supported. In general, it would be desirable if waveforms with power controlled precisely as a function of time could be reliably generated.
FIG. 1A is a diagram illustrating a CD-R laser writing waveform. The waveform begins at time to where the output power is the write power. The write power is maintained until a time t.sub.l when the power is reduced to the erase power and the waveform continues until t.sub.f when the power is reduced to a low reading level power. The length of the time interval between t.sub.l and t.sub.f is determined by the length of the mark being written. The length of the mark is expressed in terms of a time interval, T, and mark lengths vary from 3T to 11T, with 3T being the shortest mark to 11T being the longest mark. The transition at t.sub.l between the write level and the erase level is programmed and does not vary with the length of the mark being recorded or with previous or future marks. The leading edge of the waveform at t.sub.0 maybe changed by an amount .DELTA.t that is approximately equal to 1/4 T. The leading edge is shifted by .DELTA.t only when the previous mark is a 3T mark, the shortest mark allowed in a CD-R system. Thus, there is some coarse control over the leading edge of the waveform when the previous mark is a 3T mark. However, control is not provided based on future marks and precise control based on previously recorded marks is not provided.
FIG. 1B is a diagram illustrating a laser writing waveform for writing a CD-R mark after a 3T mark has been recorded. The leading edge is shifted by .DELTA.t and the remainder of the waveform is the same.
It should be noted that, as shown in FIGS. 1A and 1B, the write power and erase power are named based on the names assigned to control lines of the laser driver. The erase power therefore does not necessarily designate a power used to erase a mark. It should also be noted that the minimum power of the laser may be a biasing power that may be designated as the reading power of the laser. The power enabling signals are labeled as write power and erase power for the purpose of designating the selected power enable line that is controlled on the laser driver. It should be recognized that these names are arbitrary and that they are only meant to designate different power levels that may be specified for a laser driver.
The waveform described above for the CD-R write strategy is one example of a standard waveform used to implement a write strategy. In general, different write strategies require different waveforms to write data. It would be useful if a single chip could be used to programmably implement multiple write strategies according to instructions received from a processor. Furthermore, what is needed for more advanced write strategies such as multi-level write strategies is a method of specifying transitions more precisely. Specifically, a method is needed for altering writing waveforms to compensate for intersymbol interference and to accommodate adaptive processing techniques that may vary the writing waveform as a result of feedback.